Lexikon

The metamorphic rock is the result of the transformation of an existing rock type in a process called metamorphism, which means "change in form". The metamorphic rock is any rock derived from pre-existing rocks by mineralogical, chemical, and/or structural changes, essentially in the solid state, in response to marked changes in temperature, pressure, shearing stress, and chemical environment .The metamorphic rocks can be derived from sedimentary, igneous or another older metamorphic rock after having been subjected to heat and pressure (temperatures greater than 150 to 200 °C and pressures of 1500 bars) causing profound physical and/or chemical change. Metamorphic rocks make up a large part of the Earth's crust and are classified and named by texture, by chemical and mineral assemblage and by the characteristics of the preexisting rock. The mineral compositition of the metamorphic rocks depends on the composition of the preexisting rock, according to which the metamorphic rocks are grouped as: ultramafic, mafic, pellitic (clayee), carbonate bearing, quartz and quartz-feldspar containing rocks. Most metamorphic rocks are named based on their texture and structural features, and their mineralogy. The first step in naming a metamorphic rock is to always identify whether the rock is foliated or non-foliated. Among the foliated textures, metamorphic rocks are further classified based on how the minerals within the parent rock are affected by the changes in temperature and pressure. In general terms, increases in temperature and pressure result in different, more complex textures, and different foliated metamorphic rocks: foliated (slate, phyllite, schist, gneiss) and non-foliated. Non-foliated metamorphic rocks are usually named exclusively on the basis of mineral composition (hornfels, amphibolite, quartite). A close examination of the non-foliated rock will reveal the dominant one or two minerals within the rock. In some cases one may choose some prefix-type modifiers to attach to names to stress some important or unusual textural or mineralogical aspects. For example an “ortho”- prefix indicates an igneous parent, and a “para”- prefix indicates a sedimentary parent (orthogneiss, paragneiss). Metamorphic rocks are characterised by typical mineral assemblages according to the preexisting rock and the intensity of metamorphism. The intensity of metamorphism and the relevant metamorphic rocks are shown below:

• Very low intensity metamorphism (shale, metabasalt). Typical minerals: clay minerals, laumontite (zeolit), prehnite (zeolit).
• Low intensity metamorphism (serpentinite, chlorite shale, sericite shale). Typical minerals: sericit, pyrophyllite, chlorite.
• Medium intensity metamorphism (mica schist, marble). Typical minerals: mica, quartz, plagioclase, garnite, andalusite.
• High intensity metamorphism (eclogite, gneiss). Typical minerals: quartz, muscovite, biotite, plagioclase, kaliumfeldspar, sillimanite, staurolite.